Columbium base alloys



denum oxide is volatile at elevated temperatures.

3,163,380 COLUMBIUM BASE ALLOYS Elihu F. Bradley, West Hartford, Conn, and John J. Rausch, Evanston, .loseph it. McAndrew, Chicago, and Charles R. Simcoe, Evergreen Park, llL, assignors, by direct and ni'esne assignments, to United Aircraft (Zorporation, Hartford, (201111., a corporation of Delaware No Drawing. Filed Nov. 3, 1961, Ser. No. 149,879

I 6 Claims. (Ci. 29-494) This invention relates to novel columbium base alloys and more particularly to an alloy of columbium comprising an improved columbium aluminide compound that is resistant to oxidation at all temperatures up through 2550 F. and to methods of making and using such altoys.

The principal limitation in gas turbine technology today is the maximum turbine inlet temperature. The turbine inlet temperature is in turn limited by the temperature that the turbine vanes and blades are able to withstand without danger of failure. The best available high temperature alloys in the prior art have been nickel and cobalt base superalloys, but critical structural components such as turbine vanes and blades constructed from such alloys are limited to maximum operating temperatures of between 1600" and 1900". F.

Among the technically most important physical qualities of columbium as an alloy base are its high melting temperature (4380 F.) and its low neutron-capture crosssection. columbium is, therefore, potentially useful for fast aircraft and space flight vehicles and in nuclear reactors.

For many years it has been generally known that hi ghtemperature strength properties of metals are closely related to their melting points. Thus, metals having a high melting point also tend to have high-temperature strength potentials.

The need for structural materials for service at temper atures in excess of those attainable with present mate rials of construction has stimulated interest in the retractory metals. Until about 1957 molybdenum was considered the chief prospect for such uses. However, at the high temperature service conditions needed, molybdenum oxidizes at a catastrophic rate, principally because molyb- Because of the great problems encountered in attempting to coat molybdenum, interest has recently shifted to columbium as an alloy base for high-temperature service.

Columbium is inherently a soft, ductible, readilyfabricable material. Although its melting temperature States Patent F is about 4380 F., pure columbium becomes too Weak for structural use at temperaturesabove 1200 F. Columbium is a very reactive metal in that it dissolves large quantities of oxygen, and probably also nitrogen, on exposure to atmospheres containing even small amounts of these elements at modest temperatures.

Although columbium sufiers from oxidation, its oxide does not volatize, and it is thus potentially possible to localize oxygen attack on columbium by coating the metal. Further advantages offered by columbium base alloys as compared with molybdenum base alloys are that 3,368,386 atented Feb. 2, 196$ the compound columbium aluminide when applied to columbium base alloys by an electrodeposition process gives excelient oxidation resistance to columbium and columbium base alloys in the temperature range 2100" to 2550 F. The electrodeposition coating process there disclosed for coating aluminum on columbium base alloys to provide the compound columbium aluminide as a coat- 7 ing provides excellent control of the uniformity of the coating, particularly at edges and corners of the piece being coated. This coating process is thus effective to clad columbium base alloy articles of even intricate shapes with a uniform protective coating.

The coatings and processes for applying these con-tings as disclosed in our copending application Serial No. 150,628, filed November 7, 1961, are particularly effective in providing an oxidation resistant protective coating to columbium base alloys of the type disclosed in copending application Serial No. 358,972, filed April 6, 1964.

In'testing columbium base alloy articles that had been coated with columbium'aluminide as disclosed in our copending application Serial No. 150,628, filed November 7, 1961, we learned that although the columbium aluminide coating provided excellent oxidation resistance for the columbium base alloys at temperatures in the range 2100 to 2500" F., erratic results were obtained at temperatures below 2100 F. For example, specimens of columbium base alloys coated with columbium aluminide that were tested at a temperature of 1830 F. showed a maximum-effective life of only about eight (8) hours.

When these specimens were first subjected to prior exposure at higher temperatures, such as 2200 to 2400 F., and subsequently exposed at 1830 F., catastrophic oxidation ensued and powdering of the coating occurred within fifteen (15) minutes.

The occurrence of this phenomenon was at first attributed to the rearrangement that occurs in the coating during the early stages-of oxidation and diffusion of the coating into the base metal of the alloy. To check this theory, coated samples of columbium base alloys were diffused in a vacuum-at 1800 F. for four (4) hours prior to exposure in air; This diifusion treatment however, had no apparent effect in retarding the tendency for the coating to powder at low temperatures. The phenomenon was accordingly ascribed to an inherent property of columbium aluminide, and the general oxidation behavior of this compound was then examined.

The compound columbium aluminide (CbAl when oxidized at 1200 to '1400" F. undergoes a disintegration in which the bulk material is converted to a fine gray powder in a manner analogous to the behavior of MoSi at low temperatures as originally reported by E. Fitzer, in Molybdandisilizid als Hochtemperaturwerkstoif, Plansee Proc., 1955, Pergamon Press, London, 1956. At 1200 to 1400 F., powdering occurs within two to three hours; At 1000 F., an incubation period of about 150 hours is required, after which powdering progresses at a rapid rate.

Oxidation in the temperature range 1400 to 1800 F. has not been examined thoroughly; however, it appears that at these temperatures a combination of disintegration by powdering and normal scaling occurs.

As disclosed in our copending application Serial No. 15 0,628, filed Nov. 7, 1961, oxidation of columbium aluminide within the temperature range 2l00 to 2550 F. is at a minimum, and is sufiicient to provide excellent oxidation resistance when used as a coating on columbium base alloys for a period of hours or more. After this initial period of good oxidation resistance there is a transition to a more rapid oxidation rate in the 2100 to 2550 F. range.

At 1400 F. oxidation proceeds at a low rate until consumable or non-consumable electrodes. ing furnace using a chilled copper crucible has been used at low temperatures was .associated with an' allotropic' transformation in columbium aluminide (CbAl specimens of this material were heated in avacuum for 300 hours at temperatures varying between 8-00 and 1200 No evidence of p'owdering was observedj Although these experiments do not exclude the possibility of a lowtemperature allotrope of.CbAl being stabilized by oxygen, they do confirrn an essential relationship of environment to the disintegrationobserved; i

It was then decided to study the effect that alloying additions to the compound columbium aluminide (CbAl might have in helping to overcome the tendency of CbAl to powder at low temperatures.

Many alloy additions to CbAl were made and tested. Finally, we discovered that the addition of small amounts of silicon to columbium aluminide would have a dramatic and remarkably beneficial effect in improving the oxidation resistance ofcolumbium aluminide at all temperatures up to 2550 F.

In view ,of the foregoing description, it is a primary object of this invention to provide an oxidation resistant columbium aluminide compound that has excellent resistance to oxidation at all temperatures up through 2550 F.

Additional objects ofthis invention. are to provide a columbium aluminide compound for coating columbium and columbium basealloys that will provide excellent oxidation resistance for the alloys at all temperatures up through 2550 .F., that when used for a coating will be impermeable to gases, that will provide a low rate of interdiifusion between the coating and thebase metal, and that will be relatively ductile at all temperatures.

A further object of this invention is to provide a novel columbium aluminide compound for coating columbium and columbium base alloys that comprises columbium aluminide and from about 3 to 10 atomic percent of silicon to improve the oxidation resistance of the colum bium aluminide at all temperatures up through 2550 F., and particularly to dramatically and remarkably improve the oxidation resistance of columbium aluminide in the lower temperature range of from 1200 to 2000 F.

The foregoing and other objects are realized by the alloys of this invention'that contain as essential ingredients columbium aluminide that is made up of about 75 The novel alloys or compounds of this invention can be prepared by using known melting and casting techniques. To ensure homogeneity, it may be desirable to use multiple melting. Individual melts can be melt-cast together, and the melt may be permitted to cool and solidify into a pre-determined shape.

In operation the melting can be achieved by-either an induction-type furnace or an arc furnace using either An arc, meltadvantageously.

Regardless of the type of furnace used, great care must be taken in the melting and casting operation to protect the molten metal from normal atmospheric contamination by oxygen, nitrogen, and other gases that may be present. This problem can be avoided by conducting the melting operation in a controlled atmosphere.

as described in the examples.

In preparing the metals for the charge almost any appropriate form of the metals may be used, such as shot, granular, powder, or wire. Ideally, the metals should be of the highest purity commercially obtainable. In the examples set forth below IO-gram melts were prepared and tested for oxidation at various temperatures Weight gain was measured as a function of time.

The behavior of columbium aluminide (CbAl3) at a number of temperatures below 2000 F. has already been described. It may be helpful, however, to describe the oxidation behavior of columbium aluminide (CbAl when in the pure form at temperatures of 2000 F. and 2409 F. The oxidation behavior of pure columbium aluminide (CbAl ).at these temperatures may'then be compared with the test results obtained from the examples described below.

At 2000 F. columbium aluminide (CbAl3) displays good oxidation resistance for approximately 100 hours,

examples of the invention are given below. These examples are merely. illustrative and are not to be understood as limiting the scope and underlying principles of the invention.

EXAMPLE'I A rnixture of 22.5 atomic percent of columbium, 72.5 atomic percent of aluminum, and 5 atomic percent of silicon was introduced as a charge into a water-cooled, cop- :per crucible of a non-consumable, electrode type melting furnace using tungsten electrodes.

Prior to melting the furnace chamber was evacuated four (4) times and then back-filled with argon to provide a sufliciently inert airfree controlled atmosphere. Each charge was melted a minimum of .two v(2) times, with the total number of melts needed for homogeneity being determined by operator observation. When'the charge .had been melted for the last time, the furnace was turned'ofi and the melt was allowed to cool in the argon atmosphere, discharged from the crucible, and tested as described below.

All melts were prepared as IO-gram melts and the testing procedure that was used for this example was also used for all the other examples given.

-As oxidation resistance is the primary object of this invention, the examples given were tested for oxidation resistance only, and not for other alloy characteristics, since it is known that columbium aluminide has characteristics that make itsatisfactory in other respects as a coating for columbium and columbium base. alloys.

For oxidation tests specimens were exposed to a static air-atmosphere at the temperatures and for the times described 'below. Specimens were prepared by grinding to a uniform finish with an aluminum oxide grit abrasive. After carefully weighing and measuring'all critical components, the specimens were ,exposed in pre-fired porcelain crucibles in such a manner that the maximum surface exposure was achieved. After exposure each specimen was evaluated by measuring weight gain and by visual observation of the specimens.

'had a dark grey, tightly bound oxide film that formed duirng the initial stage of oxidation and did not change appreciably even after hours. The oxidation behavior of the columbium aluminide (CbAl plus 5 atomic percent silicon'specimen of this example registered a significant improvement over the oxidation behavior of pure columbium aluminide (CbAl at 2000 F.

'Pure columbium aluminide (CbA1 begins to oxidize fairly rapidly after 100 hours of exposure at 2000 F.

Another specimen of this example was exposed to oxidation in air for 145 hours at 2400 F. After 145 hours of exposure the compound showed no appreciable weight change and maintained virtually the same high oxidation resistance that it had shown at 2000 F.

The first specimen described above after prior exposure at 2000 F. for 145 hours was exposed at 1800 F. for 40 hours with no detectable weight gain and was then exposed at 1600 F. for 40 hours, agaimwith no detectable weight gain. Similarly, the specimen above, that was exposed at 2400" F. for 145 hours wasexposed at 1800 F. for 40 hours after the prior exposure, and no detectable weight gain was registered. This specimen was then exposed at 1600 F. for 40 additional hours; again, no detectable weight gain was noted.

Another specimen of this example was then exposed, without prior high-temperature treatment at 1200 F. for 83 hours. This specimen showed no powdering or change in weight after this exposure.

From these test results it is apparent that the addition of relatively small amounts of silicon to columbium aluminide (CbA1 are remarkably beneficial in improving the oxidation resistance of columbium aluminide (CbAl at all temperatures.

EXAMPLE II A lO-gram melt of columbium aluminide (CbAl containing two (2) atomic percent silicon, with each atom of silicon replacing one atom of columbium and three (3) atoms of aluminum respectively, was prepared as described in Example 1. Static oxidation tests of this specimen were performed at 1200, 2000, and 2400 F. After hours exposure at 1200" F. this specimen showed a weight gain of 40 mg./cm. after 10 hours exposure at 2000 F., it showed a weight gain of 8 mg./cm. and after 170 hours exposure at 2400 B, it showed a weight gain of 21 mg./cm. This specimen was thus obviously unsatisfactory in the lower temperature ranges.

EXAMPLES HI THRU VIII 10-gram melts of columbium aluminide (CbAl containing, respectively, 0.5, 1, 3, 4, 6, and 8 atomic percent silicon were prepared as Examples III through VIII respectively, as described in Examples I and II. These examples were then tested for oxidation behavior in the same manner as the tests set forth in Example I. The results of these tests are presented in Table I.

alloys tested is quite acceptable at all temperatures. The minimum silicon concentration that will prevent powdering appears to be temperature dependent but occurs in the range of two (2) to three (3) atomic percent silicon.

Microscopic examination of the columbium aluminidesilicon compounds reveals the presence of second phase located at the columbium aluminide (CbAl grain boundaries. The grain boundary constituent has a eutectic microstructure and becomes continuous at about the composition columbium aluminide plus three (3) atomic percent silicon. Increased silicon concentration increases. the quantity of the grain boundary material.

In summary the compound columbium aluminide (CbAl applied by the electrodeposition of molten alumi num onto a columbium substrate has shown very promising results. The compound columbium aluminide when coated on columbium or columbium base alloys drastically improves the oxidation resistance of the base metal at temperatures in the range 2100 to 2550 F.

At temperatures below 2100 F., however, the columbium aluminide coating exhibits poor resistance to oxidation. This result was traced to the inherent oxidation behavior of columbium aluminide at these temperatures. At low temperatures (1200-l400 F.), columbium aluminide undergoes a transition in which the bulk material is converted to a fine gray powder within two to three hours. At slightly higher temperatures (1400- 1800 F.) oxidation is rapid and occurs by a combination of disintegration by powdering and normal scaling.

To overcome the low-temperature deficiencies of columbium aluminide the effect of alloying additions was studied. The discovery was made that silicon in concentrations greater than about (3) atomic percent, provides a dramatic and unexpected beneficial result in preventing the catastrophic low temperature attack of oxygen on columbium aluminide (CbAl It has also been discovered that the addition of 5 atomic percent of silicon to columbium aluminide reduces the 2400 F. scaling rate of this compound by a factor of twenty (20) This latter result was also unexpected and unusually beneficial.

The alloys of this invention comprising columbium aluminide and at least about 3 atomic percent silicon because they are oxidation resistant over all temperature ranges up through 2550 F., ofier unusually high promise as coatings for protecting columbium and columbium base alloys from oxidation at high temperatures.

The present invention in its broader aspects is not limited to the specific compositions and examples described, but also includes within the scope of the accompanying claims any departures made from such composi- T able 1 Testing Temp. of Testing Temp. of Testing Temp. of Percentage 1,200 F. 2,000 F. 2,400" F. Example of silicon No. in atomic percent Exposure Wt. gain Exposure Wt. gain Exposure Wt. gain to Air in to Air in to Air in in Hours nag/cm. in Hours mgJemfl in Hours mgJem.

170 155 40 27 170 42 40 51 40 10 8 170 '21 about 1 320 about 5 170 about 17 0 170 about 4 170 about 22 0 145 about 1 145 about 1 about 1 170 about 9 170 about 19 about 2 170 about 12 150 about 17 From the results of the foregoing tests, it is known that, at all temperatures, a minimum silicon concentration is required to eliminate the destructuve powdering reaction and rapid oxidation of columbium aluminide (CbAlg) Slightly beyond this minimum value the oxidation rate becomes lowest at each temperature. Higher silicon concentrations then produce an increase in the rate of oxidation; however, the oxidation rate of each of the higher silicon tions and examples which do not sacrifice their chief advantages.

What is claimed is:

1. A high-temperature oxidation resistant composition of matter consisting of aluminum and columbium in an atomic ratio of about 3 to 1 and from 3 to 10 atomic percent of silicon based on the sum total of atoms in the composition.

.4.. A high-temperature oxidation .sition..

3. A high-temperature oxidation resistantcornposition of matterconsisting of aluminum and colurnbium in an atomic ratio of about 3 to 1 and about 5 atomic percent of silicon based on the sum total of atoms in the composition. I I

resistant 'columbium base alloy containing at least 40 per cent by weight of columbium and having a coating comprising a composition 'of-matter consisting of aluminum and columbium in an atomic ratio of about3' to 1 and from 3 to 10 atomic per cent of silicon based'on the sum total of atoms in the composition. r

' 5; A high tcmperature' oxidation resistant columbium base alloy containing at least 40 percent by weight of 'columbium and having a coating comprising a composition of matter consisting of aluminum and columbium in an atomic ratio of about 3 to 1 and from 4 to 7 atomic percent of-silicon based on the sum total of atoms in the composition. e

' 6. A high-temperature oxidation resistant columbium base alloy containing at least 40 percent by weight of colurnbium and having a coating comprising a composi tion of matter consisting of aluminum and'columbium in an atomic ratio of about 3 to l and about'5 atomic percent of silicon based onthe sum total of atoms in the composition.

Referezices Cited in the file of this patent I UNITED STATES PATENTS g3,102,044 Joseph Aug. 27, 1963 FOREIGN PATENTS 71,099,179 Germany Feb. 9, 1 961 OTHER'VREFERENCES r Carlson: Oxidation Resistance of Aluminum Dip Coated (Aldico) Columbium Alloys, Columbium Metallurgy, vol. 10, Metallurgical Society of A.I.M.E., Interscience Publishers, New York 1,'N. Y. (from papers presented June 9-10, 1960); pp. 119-137. 

4. A HIGH-TEMPERATURE OXIDATION RESISTANT COLUMBIUM BASE ALLOY CONTAINING AT LEAST 40 PER CENT BY WEIGHT OF COLUMBIUM AND HAVING A COATING COMPRISING A COMPOSITION OF MATTER CONSISTING OF ALUMINUM AND COLUMBIUM IN AN ATOMIC RATIO OF ABOUT 3 TO 1 AND FROM 3 TO 10 ATOMIC PER CENT OF SILICON BASED ON THE SUM TOTAL OF ATOMS IN THE COMPOSITION. 